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Abstract:

The invention relates to solar cells. More particularly, the invention
relates to arrangements and methods to increase the efficiency of solar
cells.
The methods and arrangements of the invention allow to increase the
efficiency of solar cells (11, 12, 13, 14) by trapping photons into the
photovoltaic system by thermodynamic shielding based on at least one of
the following: conductive shielding, radiative shielding (20, 21, 22,
400, 410, 411) and/or convective shielding.
The best mode of the invention is considered to be a tandem solar cell of
Si (11) and InSb (12) enclosed in a vacuum container (200) to minimise
convective heat losses. Incident sunlight is focused by a lens (320) to
a diverging element (310) that disperses the sunlight into the vacuum
container (200) and on to the Si (11) layer that is facing the incident
side of sunlight. The vacuum container has reflective foil (400, 410,
411) on the inside to reflect retransmitted photons and thereby minimise
radiative losses. InSb layer (12) is behind the Si layer (11). The
semiconductors are suspended with metal wires, minimising conductive heat
losses, which may include the electrical contacts to the load (500) or
the DC inverter.

Claims:

1. A solar cell module, comprising: a solar cell arranged inside a
housing, the solar cell comprising at least one intersubband
semiconductor layer; and a reflective cavity arranged inside the housing,
the reflective cavity housing the solar cell and being configured to
focus incident photons and reflect radiated photons onto the solar cell
such that the intersubband semiconductor layer absorbs the incident and
reflected photons.

2. The solar cell module as claimed in claim 1, wherein the solar cell is
surrounded by a vacuum or gas at low pressure.

3. The solar cell module as claimed in claim 1, wherein the solar cell is
suspended by thin wires or other conduction insulation.

4. The solar cell module as claimed in claim 1, wherein solar cell is
surrounded by a reflecting foil to reflect radiation from the solar cell
back to the solar cell.

5. The solar cell module as claimed in claim 1, wherein the photovoltaic
cell is at least one of within or behind a transparent membrane or a
casing.

6. The solar cell module as claimed in claim 5, wherein said membrane or
casing comprises a vent.

7. The solar cell module as claimed in claim 1, further comprising a
thermostat.

8. The solar cell module as claimed in claim 1, wherein the solar cell is
connected to at least one of a vacuum pump or a load.

9. The solar cell module as claimed in claim 1, wherein the solar cell
module comprises a plurality of semiconductor layers arranged in
spherical layers, one on top of the other.

10. (canceled)

11. The solar cell module as claimed in claim 1, further comprising
radiative shielding that comprises at least one of any of the following:
a mirror, a reflector or an antenna.

12. The solar cell module as claimed in claim 1, wherein the reflective
cavity comprises reflective shielding to reflect unabsorbed photons back
to the solar cell.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a division of copending application Ser. No.
12/678,536 filed on Mar. 17, 2010; which is the 35 U.S.C. 371 national
stage of International application PCT/EP08/61523 filed on Sep. 2, 2008;
which claims priority to Finnish application 20070743 filed on Oct. 1,
2007. The entire contents of each of the above-identified applications
are hereby incorporated by reference.

TECHNICAL FIELD OF INVENTION

[0002] The invention relates to solar cells. More particularly, the
invention relates to arrangements and methods to increase the efficiency
of solar cells.

BACKGROUND

[0003] The efficiency of solar cells is currently so low, that solar
energy has not been competitive against fossil fuels during low energy
prices. Due to this many technologies have been proposed to make solar
cells more efficient and thus increase the competitiveness of solar
energy in the global marketplace.

[0004] EP 1724841 A1 describes a multilayer solar cell, wherein plural
solar cell modules are incorporated and integrally laminated, so that
different sensitivity wavelength bands are so that the shorter the centre
wavelength in the sensitivity wavelength band is, the more near the
module is located to the incidental side of sunlight. This document is
cited here as reference.

[0005] Optical concentrators, such as lenses and mirrors are known in the
art, please see Solar Electricity, Thomas Markvart, 2nd Edition,
ISBN 0-471-98852-9 p. 234 which is cited here as reference. Solar cells
in space have been known to produce more power than on Earth. The prior
art understanding among professional physicists is that this is because
the solar spectrum is different in space with more photons available, due
to the lack of the filtrating effect of the atmosphere.

SUMMARY OF THE INVENTION

[0006] The invention under study is directed towards a system and a method
for effectively improving the efficiency of solar cells. A further object
of the invention is to present the most efficient solar cell for energy
production known to man. An even further object of the invention is to
reduce the unit production cost associated with solar photovoltaic
setups.

[0007] According to one aspect of the invention, the solar cell comprises
a photovoltaic cell, typically of semiconductor material. In this
application a semiconductor layer or material is construed as a layer of
any material or comprising any material capable of experiencing the
photoelectric effect. A photovoltaic cell is construed as a cell capable
of experiencing the photoelectric effect and producing current thereby. A
solar cell is construed as such photovoltaic cell when the input light or
the intended input light originates from the Sun. A photovoltaic cell is
therefore composed of at least one layer capable of experiencing the
photoelectric effect, i.e. semiconductor layer as defined in this
application. Due to this, solar cell, photovoltaic cell and semiconductor
layer are used interchangeably in some parts of this application.

[0008] In some embodiments at least one semiconductor is a quantum cascade
semiconductor or a quantum well infrared semiconductor. A quantum cascade
semiconductor is understood as any semiconductor that exhibits
intersubband transitions in addition to and/or instead of interband
transitions. One practical example of a quantum cascade semiconductor is
the quantum cascade laser and one practical example of the quantum well
infrared semiconductor is the Quantum Well Infrared Photodetector. These
examples are described in more detail in the Wikipedia article "Quantum
cascade laser" and the NASA article "Inexpensive Detector Sees the
Invisible, In Color", which are incorporated in this application herein
as reference. In a quantum cascade semiconductor, quantum well infrared
semiconductor or in fact any intersubband semiconductor the photons are
absorbed and excite electrons into intersubband transitions resulting in
the electrons moving from lower energy subbands to higher energy
subbands. The excited electrons are then harnessed as photocurrent in
accordance with the invention.

[0009] One aspect of the invention involves a solar cell with a
semiconductor layer with a natural band gap NB1. The incoming photons
therefore experience a NB1 band gap, referred here to as the natural band
gap. Photons with E>NB1 will be absorbed into the band gap NB1, and
the electron in the semiconductor valence band will get excited onto the
conduction band thus resulting in photocurrent. The photon population
that is not absorbed consists of photons with E<NB1 that had too
little an energy to get absorbed. Additionally the photons that got
absorbed with E>NB1 will only give out an energy equal to the natural
band gap NB1 in the excitation process of the electron to the
photocurrent. The remaining energy E-NB1 will be emitted as a secondary
photon of energy E2=E-NB1 or multiple photons among which energy E2=E-NB1
is distributed in accordance with the laws of conservation of energy and
momentum and quantum mechanics. These two groups, photons with E<NB1
and E2=E-NB1 belong to the secondary photon population.

[0010] It is also true that some of the photons with E>NB1 will not get
absorbed, because they are simply unable to find the valence electron and
interact with it. This fraction is not influenced by the band gap,
however. The number of missed E>NB1 is a function of the concentration
of the ion/atom/molecule species with the valence electron N1 and the
scattering cross section of this electron. Also lattice packing density
of the material, temperature etc. may have some effect. In one aspect of
the invention the fraction of missed E>NB1 in the semiconductor layer
is minimised. This group of unabsorbed photons with E>NB1 is further
added to the secondary photon population.

[0011] It is also possible that the remaining energy E-NB1 is distributed
as phonons in accordance with the laws of conservation of energy and
momentum and quantum mechanics. Phonon is the vibrational quanta of the
energy associated with mechanical heat vibration, in a similar fashion to
photon representing the quantum of light or other electromagnetic
radiation. As E is distributed to phonons, the semiconductor material
heats up, because the atoms in the lattice start vibrating stronger (i.e.
with more phonons or with higher quanta phonons). The solar cell heats
up, and it is said in prior art terms that solar energy is wasted as
heat.

[0012] The objective of the invention is to collect this allegedly wasted
energy as electricity. Firstly, it is to be realised in accordance with
the invention that the solar cell cannot heat up indefinitely. This is
because eventually the solar cell must be in thermal equilibrium with its
surroundings, in accordance with the laws of thermodynamics (zeroth law).
The solar cell obtains thermal equilibrium with its surroundings by
essentially two means; 1) it radiates photons as heat, or 2) it exchanges
phonons with its surroundings. In practice 2) involves the phonon quanta
from the solar cell being transferred to the surrounding air, by the
means of surrounding air molecules obtaining higher phonon quanta. Phonon
and photon are interchangeable quanta in accordance with the laws of
conservation of energy and quantum mechanics: A vibrating lattice with
phonon quanta Epn may emit a photon with Ept, provided
Epn>Ept, and be left with phonon energy Epn-Ept.
This analysis holds for both interband and intersubband semiconductors
mutatis mutandis.

[0013] It is currently easier to collect the energy of photons as
photocurrent in accordance with the invention. Therefore it is an object
of the invention to provide thermodynamic conditions for the solar cell
in which secondary photon production and capture is maximised and
secondary phonon production, i.e. temperature of the solar cell is
controlled accordingly. Firstly it is in accordance with the invention to
prevent the transfer of energy from the solar cell by means of phonon
transmission. This is because when the gas surrounding the solar cell
heats up, this energy is literally lost as `hot air`. Therefore any air
or gas that is in contact with the solar cell is removed entirely or
partially in accordance with the invention in one aspect of the
invention. In one aspect of the invention, the solar cell is placed in a
vacuum, and therefore heat loss by convection is minimised in this
embodiment. The solar cell should not be in contact with any solid body
either, apart from electric wires etc. needed for current collection. All
contact with solid bodies should be heat insulated in the best way
possible, thereby avoiding heat transfer by phonon-phonon interaction at
a solid surface, i.e. conduction. When heat loss by conduction and
convection is minimised or avoided, the solar cell will become hot in
accordance with the invention.

[0014] By convective shielding we mean any deliberate design aimed at
inhibiting the heat exchange by convection of the solar cell with its
surroundings. Some designs in accordance with the invention may include
placing the solar cell in vacuum, or surrounding it with very thin gas,
for example.

[0015] By conductive shielding we mean any deliberate design aimed at
inhibiting the heat exchange by conduction of the solar cell with its
surroundings. Some designs in accordance with the invention may include
insulating the solar cell with any material of low thermal conductivity,
for example Styrofoam, rubber or disordered layered WSe2 crystals or
any other material or purpose built material for heat insulation.

[0016] The only way in which the solar cell can pursue thermal equilibrium
is now radiation, which produces the photons we desperately prefer over
the phonons. It is an object of the invention to collect these photons as
photocurrent and we should not allow them to radiate away in accordance
with the invention. In accordance with the invention the radiating solar
cell is radiatively shielded for example by a reflecting foil that
reflects the resulting radiated photons back to the solar cell.

[0017] By radiative shielding we mean any deliberate design aimed at
inhibiting the heat exchange by radiation of the solar cell with its
surroundings. Some designs in accordance with the invention may include
shielding the solar cell with reflective metal foil, or mirrors aimed at
reflecting the retransmitted photons.

[0018] These photons may recombine with other photons of this
retransmitted photon population, or the retransmitted photon population
may recombine with the aforementioned secondary photon population.

[0019] Radiative shielding in the context of the invention is not limited
to any specific wavelength regime or design choice. In some embodiments
the radiative shielding may be realised with a mirror, multilayer mirror
that has several layers each with a different reflection-wavelength
function, or an antenna. The multilayer mirror may have several
reflecting layers. The antenna may be mechanical or electromagnetically
generated, for example with a magnetic field.

[0020] From prior art it is known that high T reduces the semiconductor
solar cell efficiency, whereas a high irradiance increases it. It is an
object of the invention to maximise irradiance whilst impeding the heat
loss mechanism related to high T in semiconductors. When higher
irradiance is achieved, and heat losses associated with the high T are
inhibited, greater efficiency will result for the solar cell.

[0021] In a further aspect of the invention the solar cell is a tandem
cell. For example a silicon layer at natural band gap of roughly 1 eV
captures a relatively good efficiency from the incoming solar spectra,
whereas Sb (antimony) has a low band gap of about 0.3 eV and InSb (Indium
Antimony) an impressive 0.17 eV, both applicable to converting
retransmitted photons into photocurrent. This way, a tandem cell can be
designed so that there is one designated semiconductor for incoming solar
radiation (i.e. silicon in this case), and one semiconductor for the
entrapped photons (i.e. InSb in this case).

[0022] In a further aspect of the invention the solar cell comprises
electrodes providing an ambient voltage, and thereby altering the natural
band gap NB1 to an apparent band gap B1, which is typically lower but may
also be higher, as outlined in patent application FI 20070264 "Active
solar cell and method of manufacture", of the applicant. The solar cell
setup of this application under study could be designed with the method
described in FI20070801, Mikko Vaananen, "Method and means for designing
a solar cell" of the applicant that is hereby incorporated in this
application and also referenced here.

[0023] A solar cell in accordance with the invention comprises at least
one photovoltaic cell and is characterised in that, the photovoltaic cell
is convectively shielded from the surrounding atmosphere.

[0024] A solar cell in accordance with the invention comprises at least
one photovoltaic cell and is characterised in that the photovoltaic cell
is radiatively shielded from the surrounding atmosphere.

[0025] A solar cell in accordance with the invention comprises at least
one photovoltaic cell and is characterised in that the photovoltaic cell
is conductively shielded from the surrounding atmosphere.

[0026] A solar cell in accordance with the invention comprises at least
one photovoltaic cell and is characterised in that the photovoltaic cell
is convectively, conductively and/or radiatively shielded from the
surrounding atmosphere.

[0027] In addition and with reference to the aforementioned advantage
accruing embodiments, the best mode of the invention is considered to be
a tandem solar cell of Si and InSb enclosed in a vacuum container to
minimise convective heat losses. Incident sunlight is focused by a lens
to a dispersing element that disperses the sunlight into the vacuum
container and on to the Si layer that is facing the incident side of
sunlight. The vacuum container has reflective foil on the inside to
reflect retransmitted photons and thereby minimise radiative losses. InSb
layer is behind the Si layer. The semiconductors are suspended with metal
wires, minimising conductive heat losses, which may comprise the
electrical contacts to the load or the DC inverter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the following the invention will be described in greater detail
with reference to exemplary embodiments in accordance with the
accompanying drawings, in which

[0030] FIG. 2 demonstrates a more developed embodiment 20 of the solar
cell in accordance with the invention.

[0031] FIG. 3 demonstrates an embodiment 30 of the solar cell with optical
concentrators in accordance with the invention.

[0032] FIG. 4 demonstrates a more developed embodiment 40 of the solar
cell with optical concentrators in accordance with the invention.

[0033] FIG. 5 demonstrates an "onion" embodiment of the inventive solar
cell system 50 in accordance with the invention.

[0034] FIG. 6 demonstrates an open air embodiment of the inventive solar
cell system 60 in accordance with the invention.

[0035] FIG. 7 demonstrates an embodiment of the inventive solar cell
system 70 that features a tandem semiconductor structure and a tandem
reflector structure in accordance with the invention.

[0036] Some of the embodiments are described in the dependent claims.

DETAILED DESCRIPTION OF EMBODIMENTS

[0037] FIG. 1 shows a very simple embodiment of the inventive solar cell
embodiment 10. A tandem solar cell with semiconductor layers 11 and 12 is
enclosed in a casing or a membrane 200 that is transparent to solar
light.

[0040] In some embodiments the semiconductor layer 11 facing the incident
solar spectrum has the higher band gap, and the semiconductor layer 12
has the lower band gap. In some embodiments this order is reversed. The
semiconductor materials 11 and/or 12 may be arranged in any configuration
in the casing or membrane 200 in accordance with the invention.

[0041] The casing and/or membrane 200 surrounds the solar cell simply to
restrict any convective and/or conductive heat losses by holding a vacuum
or low density gas between the casing or membrane 200 and the
semiconductor layers 11 and 12, thereby forcing the semiconductor layers
11 and 12 to radiate their heat losses. Provided either one of the
semiconductor layers has a low enough band gap, this semiconductor can
collect some of the reradiated photons as photocurrent. In some
embodiments the casing or membrane 200 is very stiff in order to avoid
collapse under air pressure. In some further embodiments the casing or
membrane 200 is made of stiff transparent plastic or glass or any other
similar material in accordance with the invention.

[0042] In some embodiments the casing or membrane 200 also houses a
radiative shielding, arranged to reflect back the retransmitted photons
as explained earlier. The radiative shield should be an efficient
reflector across the band where the majority of the total solar flux
lies, between 200 nm (UV)-1500 nm (IR), and preferable above these
wavelengths as well given the performance of available materials in
accordance with the invention. In some embodiments the reflected
wavelengths can be considerably longer, for example several micrometers.
In these embodiments the reflector is either a mirror, microwave
reflector and/or an microwave antenna. In some embodiments the radiative
shielding may be realised with a mirror, multilayer mirror that has
several layers each with a different reflection-wavelength function, or
an antenna. The multilayer mirror may have several reflecting layers in
accordance with the invention The antenna may be mechanical or
electromagnetically generated, for example with a magnetic field.

[0043] It is known that quantum cascade semiconductors and/or quantum well
infrared semiconductor may feature photoelectric properties, i.e.
electron-photon absorption/emission properties at wavelengths of 2-250
micrometers. It is therefore preferable and in accordance with the
invention that an antenna and/or reflector or multiples of antennas
and/or reflectors are used to reflect photons back to the at least one
semiconductor.

[0044] In some embodiments the semiconductor or semiconductors are at the
focus or foci of these reflecting or focusing elements. In some
embodiments this reduces the cost of the photovoltaic setup. Typically
most of the cost arises from the semiconductor materials, and in these
embodiments less semiconductor material is needed. This is because the
semiconductor material at the focus or foci can be made smaller. The
reflecting and focusing materials are typically cheaper, and thus
reflecting rays back typically reduces the cost per unit watt of
photoelectric energy produced. In some embodiments a magnetic field is
used to alter the wavelength range of at least one photoelectric
semiconductor material. In other embodiments this magnetic field could be
also used with/as an antenna to reflect photons and microwaves back to
the semiconductor material.

[0045] A microwave antenna that reflects radiation at 250 micrometers
would need to have dimensions roughly equal to the length of the
wavelength. Therefore the high wavelengths of the reflected radiation
dictate the minimum unit size for embodiments 10 in some embodiments. In
some embodiments the radiative shielding is designed to reflect back the
whole secondary photon population and in order to achieve this goal, it
may have any number of layers or have any other design choices.

[0046] Because many of the high energy photons from the incident flux have
been converted to photocurrent and lower energy photons and phonons, the
emphasis on optimising the reflection properties of the radiative shield
is towards the longer wavelengths and lower energies when compared to the
raw incident solar spectrum.

[0047] In some embodiments the casing or membrane 200 houses a radiative
shielding made of any of the following: reflective foil, such as metal
foil, ultraviolet/visible/infra red mirror such as aluminium or gold
mirror or said mirror or mirror foil with opaque, vacuum-deposited
metallic coatings on low-expansion glass substrates,
Aluminum/MgF2-mirror, Aluminum/SiO-mirror, Aluminum/dielectric-mirror,
Protected Gold-mirror and/or normal mirror. The choice of the radiative
shielding material should be based on the reflectance-wavelength function
of the material amongst other practical things such as cost and
availability in some embodiments of the invention. In some embodiments it
is preferred for the reflection to be efficient up to Far-IR, or in any
case to the wavelength that equates with the smallest band gap in the
semiconductor layers 11 and 12.

[0048] Semiconductor layers 11 and 12 typically contain electrodes for
photocurrent collection. In addition, any of the semiconductor layers 11
and/or 12 may contain electrodes that are designed to actively manage the
band gap of the semiconductor material 11 and/or 12, as described in
Finnish Patent application FI20070264 of the applicant. FI20070264 is
hereby incorporated to this application. Semiconductor layers 11 and 12
may also feature several band gaps of different values in accordance with
the invention. Especially the semiconductor layer 12 may be a low band
gap material such as antimony (Sb), and electrodes can be used to produce
an ambient voltage reducing the low natural band gap to an even lower
apparent band gap, thereby capturing even more photons, especially
retransmitted photons. For example, if the semiconductor material 12 is
say InSb with a band gap of 0.17 eV, in some embodiments an ambient
voltage V=0.05 eV is provided as described in FI20070264. As a
consequence, the band gap of semiconductor material 12 might become
similar to 0.12 eV or 0.22 eV, referred to as the "apparent band gap"
depending on the sign of V. Therefore, it is possible to optimize the
band gap with regard to the photon population. If the photon population
is dominated by very low energy secondary photons and retransmitted
photons, it is preferable to lower the band gap, so that all photons with
E=0.12 eV or more have the possibility of being captured for photocurrent
generation. On the other hand, if there are plenty of photons with
E>0.22 eV around and it is more preferable to capture the maximum
energy from these photons, the band gap can be set at e.g. 0.22 eV in
accordance with the invention.

[0049] In order to save repetition it is noted that all embodiments 10,
20, 30, 40, 50, 60 and 70 may be freely permuted and changed and features
from one embodiment to the other can be transferred in accordance with
the invention.

[0050] FIG. 2 shows an embodiment 20 of the solar cell in accordance with
the invention, where the solar cell is used to power a load 500. The load
500 can be any device requiring electricity as energy, a energy storage
device such as a battery or the electric grid itself. The photocurrent is
collected from the semiconductor materials 11 and 12 to the load by
electrical wires. Ideally the wires for the photocurrent collection
should be made small and insulated, to minimise conductive and/or
convective losses.

[0051] In some embodiments the casing and/or membrane 200 also
incorporates a vent 300. In some embodiments the vent 300 may also
incorporate a thermostat. In most embodiments of the invention the solar
cell is similar to embodiment 10 explained before, and to save repetition
it is noted that all embodiments 10, 20, 30, 40, 50, 60 and 70 may be
freely permuted and changed and features from one embodiment to the other
can be transferred in accordance with the invention.

[0052] A vacuum or low density gas is provided between the semiconductor
materials 11, 12 and the casing or membrane 200 to minimise convective
transfer of heat. In some embodiments the convective shielding can be
performed with a solid material that is transparent to solar light, but
has a very low thermal conductivity in accordance with the invention.
Conductive losses of heat are minimised by minimising any physical
contact between the semiconductor materials 11, 12 and the surroundings.
If there is physical contact between the semiconductor materials 11, 12
and the surroundings the contact should be made with material of low
thermal conductivity and/or the contact should be well thermally
insulated. For example, the semiconductor materials 11, 12 making up at
least one photovoltaic cell can be suspended in the vacuum by thin wires.
In some embodiments these wires have a dual use of conducting the
photocurrent out of at least one photovoltaic cell 11, 12.

[0053] These aforementioned restrictions force the photovoltaic cell 11,
12 to release more excess heat as radiation as explained before. The
photovoltaic cell thus radiates photons outward. This radiation typically
takes a spectrum similar to the so called "black body"-spectrum,
theoretically described by the Planck Radiated Power Density-equation,
known to professional physicists and available in literature. Therefore
in some embodiments of the invention the casing or membrane 200 is
covered by a reflective foil or cover from the inside, thereby providing
radiative shielding.

[0054] With all or some of the aforementioned shieldings in place, the
temperature of the photovoltaic cell 11, 12 should climb. For some
semiconductor materials this leads to a drop in performance and
efficiency. However, as the temperature climbs, also the irradiance
within the casing or membrane 200 is increased. The power output of the
photovoltaic cell naturally increases with irradiance. It is therefore
important in accordance with the invention to optimise the thermodynamic
conditions of the photovoltaic cell to maximise power output, lifetime
and other production characteristics of the solar cell. If the
temperature rises too high, the vent 300 can be used to let air flow into
the casing or membrane 200, thereby convectively cooling the photovoltaic
cell 11, 12. The vacuum pump 600 may be used to pump air out of the
casing or membrane 200, thereby providing further conductive and
convective insulation in some embodiments of the invention. In some
embodiments the pump 400 can also be operated such that air is pressed
into the casing or membrane 200 to provide for cooling in emergency or
other situations.

[0055] Let's see whether the invention makes any sense in practice by
means of a simple quantum mechanical calculation. One can reasonably
expect that the maximum temperature the semiconductor material can reach
will be roughly 1700K (the melting point of silicon), before it starts to
melt, even though clever material choices could bring it higher and other
choices lower in accordance with the invention. The black body spectrum
is therefore given by the Planck's Law with T=1700K and kT=2.3*10 (-20)
J. The maximum intensity at this temperature is given by the Wien's
displacement law Tλ(max I)=2.898*10 6 nanometer Kelvin. This yields
roughly 1.7 micrometers as the wavelength. Quite clearly at least the
intersubband semiconductor materials, if not interband materials, can
harness this radiation as photoelectric energy in accordance with the
invention! Even more simply kT=hf yields a wavelength of about 8
micrometers that is well and safely within the current range of
intersubband semiconductor materials.

[0056] It is therefore possible to realise the "irradiance cradle" of the
invention with optical feedback and photoelectric conversion in the
energy domain attributed to the radiation spectrum of a hot solar cell.

[0057] It should be noted that any of the embodiments of the invention can
be realised in any physical size or dimensions in accordance with the
invention. It should also be noted that any number of semiconductor
materials 11, 12, and/or any number of photovoltaic cells built from
these semiconductor materials or other materials can be used to realise
the solar cell system in accordance with the invention. It should further
be noted that all embodiments 10, 20, 30, 40, 50, 60 and 70 may be freely
permuted and changed and features from one embodiment to the other can be
transferred in accordance with the invention.

[0058] FIG. 3 displays an embodiment 30 of the inventive solar cell, where
optical concentration devices and radiative, conductive and convective
shielding are used to maximise photon entrapment in the casing and/or
membrane 200.

[0059] The solar cell system comprises a focusing element 320, such as a
lens or a mirror that is used to focus the incident solar light to a
smaller area, thereby increasing flux in that area. The focused solar
light is directed to an opening into the casing 200 for housing the
photovoltaic cell system with semiconductor materials 11, 12. In some
embodiments this opening may be installed with a ray diverging element
310 that spreads the solar light from the focused area to a wider area as
the solar light passes through it. In some embodiments the diverging
element 310 is a prism, mirror or a lens. Typically the elements 320, 310
are arranged so that a maximum photon collection area is obtained, and
the photons are spread out across the entire surface of the photovoltaic
cell 11, 12, in this case the semiconductor layer 11 which is on the
incidence side.

[0060] Photons are thus collected from a large area and focused onto the
semiconductor layer 11. The photons that do not interact with
semiconductor layer 11 to produce photocurrent at the respective band gap
or band gaps of semiconductor layer 11 are either scattered to the walls
of the casing, absorbed as phonons into the lattice structure of the
semiconductor layers, or pass through to the second semiconductor layer
12. Solar photons that failed to interact with a semiconductor layer 11,
12 or retransmitted photons that failed to interact with a semiconductor
layer 11, 12 will eventually reach the wall 400, 410, 411 of the casing
200. This wall typically comprises reflective shielding, such as mirror
foil, and the photon is reflected back. Most probably the reflected
photon will again be directed to the photovoltaic system 11, 12 and will
have a new chance to be converted into photocurrent. Provided the
reflectance of the casing 200 walls is high enough, a photon can be
bouncing between the walls for several casing crossing distances having
several chances of being turned into photocurrent in accordance with the
invention. This holds also for the case when the casing walls 200 form a
reflecting antenna in some embodiments.

[0061] A vacuum or low density gas is provided into the casing 200 to
minimise convective and conductive losses by phonon transfer and gas
motion. Conductive heat losses are minimised by suspending the
photovoltaic cell or cells in the vacuum or thin gas by wires, which are
preferably thermally insulated in accordance with the invention. Heat is
liberated from the photovoltaic cell thus mainly by radiation, and the
reradiated i.e. retransmitted photons are transmitted against the
reflective wall of the casing 200, from which they are typically
reflected back to the semiconductor layers 11, 12 for a further try to
convert to photocurrent in accordance with the invention. In some
embodiments it is possible for the photons to escape the casing 200 by
exiting via the opening housing the diverging element 310, but because
the area of the opening is small in comparison to the total wall area of
the casing 200, the probability for escape per photon is small, and on
average most photons should be directed to the semiconductor materials
11, 12 several times before getting any statistical chance of escaping
from the casing 200. Thereby the irradiance in the casing 200 and onto
the semiconductor layers 11, 12 is maximised in accordance with the
invention.

[0062] It should be noted that any of the embodiments of the invention can
be realised in any physical size or dimensions in accordance with the
invention. It should also be noted that any number of semiconductor
materials 11, 12, and/or any number of photovoltaic cells built from
these semiconductor materials or other materials can be used to realise
the solar cell system in accordance with the invention. It should further
be noted that all embodiments 10, 20, 30, 40, 50, 60 and 70 may be freely
permuted and changed and features from one embodiment to the other can be
transferred in accordance with the invention.

[0063] FIG. 4 shows an embodiment 40 of the inventive solar cell system
with optical concentration devices and a photon entrapment geometry and
design where a vent 300 and vacuum pump 600, and a thermostat in some
selected embodiments, are arranged to control the temperature and the
irradiance in the casing 200. The photovoltaic cell 11, 12 is used to
power a load 500, which can be a machine, energy storage system, such as
a battery or a fuel cell, or a electricity grid. Irradiance and
temperature in the casing may also be altered by adjusting the photon
collection area of the focusing element 320. If a critical temperature is
reached at any point in the system, the thermostat will release cooling
air into the casing 200 in some embodiments.

[0064] In some embodiments of the invention the distance between elements
320, 310 is minimised to make the system as flat as possible. The
distance between the casing walls 400, 411, 412 and the semiconductor
layers 11, 12 can also be minimised, even to zero, in accordance with the
invention.

[0065] It should be noted that any of the embodiments of the invention can
be realised in any physical size or dimensions in accordance with the
invention. For example a solar energy farm with solar cell systems 40 of
size hundreds of meters across could be designed in accordance with the
invention, whereas smallest systems 40 could be far smaller than the size
of the human palm of a hand. In some embodiments thin films of few micron
or some nanometers are possible in accordance with the invention.

[0066] It should also be noted that any number of semiconductor materials
11, 12, and/or any number of photovoltaic cells built from these
semiconductor materials or other materials can be used to realise the
solar cell system in accordance with the invention. It should further be
noted that all embodiments 10, 20, 30, 40, 50, 60 and 70 may be freely
permuted and changed and features from one embodiment to the other can be
transferred in accordance with the invention.

[0067] FIG. 5 presents an embodiment that is perhaps the best but most
demanding embodiment of the invention. Solar light is collected and
focused by the focusing element 320 to the diverging element 310 as
before, and the solar light is directed in to the spherical casing 200.
The casing 200 is convectively insulated by a vacuum or thin gas as
before. The internal casing walls have radiative shielding, such as
mirror foil 400 as described before. At the focal point of the radiative
shielding 400 or at the centre of the spherical casing 200 is the
photovoltaic system, comprising one or more photovoltaic cells. The
semiconductor layers 11, 12, 13 and 14 make up the photovoltaic cells. In
some embodiments the semiconductor layers 11, 12, 13, 14 have different
band gaps. In some embodiments of the invention it is possible that the
layer 11 has the highest band gap, 12 the next highest band gap, 13 has a
band gap lower than 12, and 14 has the lowest band gap. However, this
order of band gaps could be reversed, or indeed the semiconductors may be
arranged in any order in accordance with the invention.

[0068] In one exemplary embodiment semiconductor layer 11 is made of a
GaN-layer, preferably with a band gap of 3.4 eV in accordance with the
invention. The semiconductor layer 12 is a InGaP-layer at approximately
band gap 1.93 eV in this embodiment, and the semiconductor layer 13 is a
polycrystalline silicon at band gap of 1.1 eV, and the fourth
semiconductor layer 14 is typically of InSb at a band gap of 0.17 eV, for
example. All solar photons are first focused to the photovoltaic system,
top semiconductor layer 11. Some high energy photons are absorbed at the
3.4 eV band gap, other photons are not, and some photons leave a photon
belonging to the secondary photon population as defined earlier and in
FI20070264. These photons enter semiconductor layer 12 and may get
absorbed by the 1.93 eV band gap, however, some photons are again not
absorbed, and some photons are left as excess from Eph-1.93 eV,
belonging to the secondary photon population of this layer. The resulting
photons then enter the semiconductor layer 13 and may get absorbed by the
1.1 eV band gap, however, some photons are again not absorbed, and some
photons are left as excess from Eph-1.1 eV, belonging to the
secondary photon population of this layer. Lastly, the resulting photons
then enter the semiconductor layer 13 and may get absorbed by the 0.17 eV
band gap, however, some photons are again not absorbed, and some photons
are left as excess Eph-0.17 eV, belonging to the secondary photon
population of this layer.

[0069] As we can see all photons above 0.17 eV have several chances of
getting absorbed as photocurrent, when the radiative shielding 400
reflects the photons from every position of the internal wall of the
casing through the centre of the spherical photovoltaic system comprising
layers 11, 12, 13, 14. In some embodiments the diverging element 310 is
replaced by a focusing element, focusing the solar light rays through the
centre of the spherical photovoltaic system comprising layers 11, 12, 13,
14. 0.17 eV translates to an energy of 2.7*10 -20 J and a wavelength of
about 7 microns, which should also easily be handled by for example any
quantum cascade semiconductor material in accordance with the invention
or also quantum well infrared semiconductor for that matter, and perhaps
even some interband semiconductor materials.

[0070] In some embodiments any of the layers 11, 12, 13, 14 may comprise
electrodes supplying an ambient voltage altering the band gaps, as
described in the Finnish patent application FI20070264 of the applicant,
which is incorporated into this application. Especially in some
embodiments, at least one of the band gaps can be pulled to a lower level
than even 0.17 eV by providing an ambient voltage that reduces the
natural band gap. In these ways, even very low energy IR- and/or
microwave-photons may be captured as photocurrent in some embodiments of
the invention.

[0071] The embodiment 50 boosts the solar cell system efficiency by
entrapping photons into the casing 200, and adjusting their optical path
so that they will go through the semiconductor layers 11, 12, 13, 14, or
at least some of them, several times. For example if the radiative
shielding 400 has a reflectance of 90%, 50% of the flux will experience
an effective optical path increase by a factor of 6 or more (0.9 6=0.53).
In other words, even after 6 reflections and 6 crossovers across the
casing 200, 53% of the photon flux will still be reflecting, if not
already absorbed. Therefore the reflectance of the radiative shield 400
should be as high as possible in accordance with the invention in some
embodiments. As more photons get trapped, the irradiance increases. As
the photovoltaic 11, 12, 13, 14 system is thermally insulated by
conductive, convective and radiative shielding, the temperature
increases. Therefore the casing 200 forms a "hot irradiance cradle" for
the photovoltaic cells 11, 12, 13, 14, producing electric energy from
sunlight with high efficiency over time- and area-integrated available
sunlight. The "irradiance cradle" architecture of the invention is also
of very low production cost, because there is less semiconductor material
needed in this architecture per unit watt of power produced.

[0072] In some other embodiments the photovoltaic system comprising the
layers 11, 12, 13 14 may not be arranged in an "onion" style
architecture, i.e. having a layer on layer, but differently. It is
possible that the photovoltaic system comprises several structures in a
"Morula" structure (like a raspberry or cloudberry), with spots of
different semiconductor material 11, 12, 13, 14 in different places of
the photovoltaic system. Also, the photovoltaic system need not be
spherical, it may be of any shape, conical, square, triangular, or indeed
of any shape in accordance with the invention.

[0073] In some embodiments the inner most layer is sensitive to the
smallest energy photons, i.e. an intersubband semiconductor layer such as
a quantum cascade semiconductor and/or quantum well infrared
semiconductor would be at the core 14 of a spherical photovoltaic system.
In some embodiments of the invention the spherical semiconductor is
realised so that consecutive semiconductor layers are grown on top of a
"protoball". If the protoball were left inside, it would be the core 14
with a quantum cascade semiconductor at layer 13 in some embodiments. The
highest energy semiconductor would be at 11 in some embodiments, but
naturally the semiconductor layers 11, 12, 13, 14 may have bandgaps that
are interband or intersubband in any order in accordance with the
invention. Similarly other shapes for the semiconductors may be realised
by growing consecutive layers on some other protoshape in accordance with
the invention. Any crystal growth methods mentioned in this application,
and others, may be used in accordance with the invention. Alternatively
in some embodiments many shapes may be assembled from for example square
semiconductor elements. In most embodiments the most important thing is
that the depletion region of the p-n junction obtains the maximum
exposure to the incident radiation, other factors are typically only
design details in comparison to this parameter in accordance with the
invention. In some embodiments of the invention the p-n junctions are
realised radially in the spherical solar cell. In some embodiments the
electrodes are simply realised on radial structures in the spherical
solar cell, which is typically at the focus of any reflecting and/or
focusing elements in accordance with the invention.

[0074] The photovoltaic system comprising the semiconductor materials 11,
12, 13 and/or 14 is used to drive the load 500, which may be a machine,
energy storage system or an electric grid, in some embodiments. The
vacuum or the content of gas in the casing 200 is typically controlled by
a vacuum pump 600 and a vent 300, which may comprise a thermostat as
described before. The electrodes collecting current from semiconductor
layers 11, 12, 13, 14 may be realised in any feasible way, and the said
electrodes are connected to the load 500 by conducting electrical wires.
The electrodes providing any ambient voltage or collecting photocurrent
are typically manufactured and/or grown into the semiconductor layers 11,
12, 13, 14 by screen printing, as explained in Solar Electricity, Thomas
Markvart, 2nd Edition, ISBN 0-471-98852-9 or by any other method in
accordance with the invention. Alternatively, they could be implemented
as a separate layer on top the semiconductor layers 11, 12, 13, 14 in
some embodiments. In this embodiment the conductor layer is typically
transparent in accordance with the invention. The electrical contacts
and/or the electrodes preferably occupy the minimum area when meshed with
the semiconductor layers 11, 12, 13 and/or 14.

[0075] Solar power is typically DC current, so the load 600 may comprise
an AC/DC inverter in some embodiments.

[0076] It should be noted that any of the embodiments of the invention can
be realised in any physical size or dimensions in accordance with the
invention. It should also be noted that any number of semiconductor
materials 11, 12, 13, 14 and/or any number of photovoltaic cells built
from these semiconductor materials or other materials can be used to
realise the solar cell system in accordance with the invention. It should
further be noted that all embodiments 10, 20, 30, 40, 50, 60 and 70 may
be freely permuted and changed and features from one embodiment to the
other can be transferred in accordance with the invention.

[0077] FIG. 6 presents an outdoor embodiment 60 of the invention with only
radiative shielding. At least one mirror 700, 701, 702 and/or 703 directs
solar light to the photovoltaic cell system 11, 12, 13, 14 at the centre
or focus of at least one mirror 700, 701, 702, 703. The system can be
realised for example on a field 800. In this embodiment there is no
convective or conductive shielding, because the heat in the photovoltaic
system can be freely disseminated into surrounding air. There is however,
radiative shielding in accordance with the invention. Suppose solar light
is directed from mirror 700, the photons may get absorbed in any of the
semiconductor layers 11, 12, 13 and/or 14 with different or same band
gaps. Some of the photons do not get absorbed as explained before. These
photons may pass through the layers 11, 12, 13, 14 to the mirror 703 only
to be reflected back to the photovoltaic cell system 11, 12, 13, 14.
Likewise the photons scattered to other mirrors 701 or 702 are reflected
back to the photovoltaic cell system 11, 12, 13, 14. This way radiative
entrapment of photons to the photovoltaic system 11, 12, 13, 14 still
results, without a need to make arrangements for convective or conductive
entrapment. This embodiment of the invention is especially useful, as a
higher photon flux is obtained by reflecting unabsorbed photons between
the radiative shields, mirrors and/or antennas 700, 701, 702, 703 back
and forth and allowing new opportunities for photocurrent absorption for
these photons and/or waves in the photovoltaic system comprised of
semiconductor layers 11, 12, 13 and/or 14.

[0078] FIG. 7 shows an embodiment 70 of the invention with a tandem
semiconductor and a tandem reflector. The tandem reflector comprises at
least one microwave antenna 20 with at least one IR mirror 21 and at
least one optical mirror 22 foil covering. The optical mirror 22 is first
and reflects high energy photons, but transmits IR-photons that are then
reflected by the IR mirror 21 in some embodiments. Both mirrors 22, 21
typically transmit microwave photons that are reflected by at least one
antenna and/or reflector 20. The mirrors 22, 21 can be arranged very thin
in some embodiments, sometimes even comparable, larger or smaller in
thickness to the wavelength they are designed to transmit or reflect. All
reflectors are preferably arranged to focus the reflected photons and
waves to the photovoltaic semiconductors 11, 12, 13, and/or 14 that lie
in the centre in some embodiments. The inner semiconductor layers
typically have the lowest bandgaps and may be composed of intersubband
semiconductor materials such as quantum cascade semiconductor materials
and/or quantum well infrared semiconductors, but it is also possible that
they are composed of normal semiconductors with just low interband gaps.
However, the semiconductor layers 11, 12, 13, 14 or any combination of
them or anyone of them may be in any order, may be composed of any
material and have any bandgap in accordance with the invention. There may
also be any number of semiconductor layers and/or reflector layers in
accordance with the invention.

[0079] It should be noted that any of the embodiments of the invention can
be realised in any physical size or dimensions in accordance with the
invention. Any number of radiative shields or mirrors 700, 701, 702, 703
can be used in accordance with the invention, as long as the
configuration results in photon entrapment in the photovoltaic cell
system. Photon entrapment means here that a photon experiences the
photovoltaic system or a part of it more than once on its optical path,
as it is reflected back into the photovoltaic system, after already
having interacted with a semiconductor material 11, 12, 13, 14, but not
having been absorbed. A photon of the secondary photon population, i.e.
re-emitted photon, would also be an entrapped photon when reflected back
to the photovoltaic system 11, 12, 13, 14 after its re-emission.

[0080] It should also be noted that any number of semiconductor materials
11, 12, 13, 14 and/or any number of photovoltaic cells built from these
semiconductor materials or other materials can be used to realise the
solar cell system in accordance with the invention. It should further be
noted that all embodiments 10, 20, 30, 40, 50, 60 and 70 may be freely
permuted and changed and features from one embodiment to the other can be
transferred in accordance with the invention.

[0081] The invention has been explained above with reference to the
aforementioned embodiments and several commercial and industrial
advantages have been demonstrated. The methods and arrangements of the
invention allow to increase the efficiency of solar cells by trapping
photons into the photovoltaic system by thermodynamic shielding based on
at least one of the following: conductive shielding, radiative shielding
and/or convective shielding.

[0082] The invention has been explained above with reference to the
aforementioned embodiments. However, it is clear that the invention is
not only restricted to these embodiments, but comprises all possible
embodiments within the spirit and scope of the inventive thought and the
following patent claims.